Human locomotion, such as walking and running, is commonly described in terms of gait. Gait is a cyclical or reoccurring pattern of leg and foot movement, rotations, and torques that creates locomotion. Due to the repetitive nature of gait, gait is typically analyzed in terms of percentages of a gait cycle. A gait cycle is defined for a single leg beginning with the initial contact of the foot with a surface such as the ground. The initial contact of the foot on the ground is referred to as a heel strike. The conclusion of a gait cycle occurs when the same foot makes a second heel strike. A gait cycle can be divided into two phases: stance phase and swing phase. Stance phase describes the part of the gait cycle where the foot is in contact with the ground. Stance phase begins with heel strike and ends when the toe of the same foot leaves the ground. Swing phase describes the part of the gait cycle where the foot is in the air and not in contact with the ground. Swing phase begins when the foot leaves contact with the ground and ends with the heel strike of the same foot. For walking gait speed, stance phase typically describes the first 60% of the gait cycle, while swing phase describes the remaining 40% of the gait cycle.
Individuals have unique gait patterns. Energy or metabolic expenditure during an individual's gait depends on several factors including, body mass, stride length, step rate, and other physical and environmental factors. Individuals have physical and metabolic limits, which determine the speed and distance an individual can travel on foot. Decreasing the metabolic cost for an individual's gait allows the individual to run faster or travel for a longer distance while minimizing the energy expended by the individual.
Fatigue and injury can result from overuse or from strenuous activity, such as long distance walking and load carrying. Carrying significant loads over long distances and time periods can lead to fatigue and cause musculoskeletal injuries. Various types of jobs require people to carry loads. Military personnel are considered particularly at risk for fatigue and injury from carrying loads. As the quantity and complexity of gear used in military duty has increased, the weight of loads carried by military personnel has also increased. Many soldiers carry a variety of devices, such as night goggles, global positioning systems (GPS), body armor, and other gear. Although maximum loads are recommended, the recommended maximums are typically exceeded. Typical loads carried by soldiers can range between 45 kilograms (kg) to 60 kg or more. Soldiers often carry the loads for long distances while marching on foot.
The relationship between distance traveled and the rate of metabolic energy expended is exponential in nature. The metabolic cost of gait depends on the speed of gait and the weight of a load carried by the individual. When carrying a heavier load, the speed of a march is decreased in order to avoid fatigue. Fatigue has been shown to have detrimental effects on individuals who carry the heavy loads. Fatigue is known to increase likelihood of acute injury by raising the potential for trips and falls. Fatigue can also affect mental focus, reduce situational awareness, and negatively impact overall physical and mental performance. Non-combat related injuries caused by carrying significant loads are also a problem. Long term and chronic overuse injuries account for a significant amount of injuries for soldiers.
Various types of structures and exoskeletons have been proposed to support or lessen loads carried. Current load assistance structures are known to perturb the user's gait and negatively affect metabolic expenditure. Interference with gait creates inefficiencies in energy transfer by altering the fluidity of the gait motion. Disruption of the natural gait step causes an increase in metabolic cost. Altering an individual's gait dynamics also increases the likelihood of acute and chronic injury.